1. Field of the Invention
The present invention relates to a disk array apparatus having parity disks and, more particularly, to an optical disk array apparatus using a plurality of optical disks as information recording media.
2. Description of the Related Art
In recent years, the amount of data managed in an office have increased. In order to manage a large amount of data, various electronic filing systems using an optical disk array apparatus have been proposed.
This optical disk array apparatus is constituted by a plurality of optical disk drives, cells for housing a plurality of optical disks, and an accessor for moving the optical disks between the optical disk drives and the cells.
In order to manage a large amount of data with high reliability at high speed, it is desirable that data to be stored in the disk array apparatus should have a certain level of redundancy and should be distributed and stored in a plurality of disks in units of blocks.
For example, in a RAID (Redundant Array of Inexpensive Disks) constituted by N disks, parity data corresponding to data at the same block number in the first to (N-1)th data disks are stored at the same block number in the Nth parity disk. A set of these N disks including the data disks and the parity disk is called a group.
When consecutive data are to be read out from the above group, the respective data disks in which the corresponding blocks are stored can be simultaneously accessed. This allows a high-speed read operation.
If an error occurs in one data disk in a read operation, recovery from the error can be made by using identical block data in the remaining (N-1) data disks in the group to that in the disk in error. Therefore, high reliability can be ensured.
For instance, assume that A, B, C, and D are block data in specific blocks, respectively, and parity data D is determined to establish the following equation: EQU A xor B xor C=D (1)
where xor indicates an exclusive-OR operation. That is, xor is a symbol for defining a logical operation like the one shown in FIG. 3.
Assume that data disk 1 has failed, and an error has occurred in a data read operation with respect to block A of disk 1. In this case, block data A can be recovered by performing the following operation: EQU B xor C xor D=A (2)
As number N of disks constituting a group is decreased, the reliability is improved. However, if N=2, half of the total disk capacity is used for parity. That is, the proportion of the capacity, which can be actually used for data storage, to the total disk capacity is greatly reduced.
When a RAID system is constituted by an optical disk array apparatus, the greatest factors which determine an access speed are the number of times that optical disks are moved by an accessor and the moving distances. Therefore, the following considerations are important. Optical disks with high probabilities of access are preferentially left in optical disk drives. If this is not possible, such optical disks are housed in housing cells near optical disk drives.
Even with such considerations, if a read error occurs, it takes much time for recovery processing because data is recovered by accessing all the optical disks in the group except for the disk in which the error has occurred. Especially, as the number of times that optical disks required for recovery are loaded into optical disk drives increases, this processing time increases.
That is, the following trade-offs occur. As the number N of disks constituting a group increases, the total storage capacity increases, but recovery processing at the occurrence of an error is slowed down. In contrast to this, as the number N of disks constituting a group decreases, recovery processing at the occurrence of an error is speeded up, but the total storage capacity which can be actually used for data storage is reduced.
number N of disks in group small.fwdarw.large PA1 number of disks required for recovery processing small.fwdarw.large PA1 total storage capacity of group small.fwdarw.large PA1 reliability high.fwdarw.low